Previous studies with animals have shown that blood pressure response to high sodium intake is correlated with sustained respiratory suppression resulting from aversive behavioral conditioning. The present project seeks to identify individual differences in tonic ventilation of human subjects as a predictor of individual blood pressure response to high sodium intake. A respiration monitor has been devised which records tidal volume and breath-rate of ambulatory subjects in the natural environment. The monitor integrates methodology of inductive plethysmography with a portable microprocessor which can record respiratory measures continuously for days. Studies have shown significant differences between individuals in 24-hour breathing rate, tidal volume, and minute ventilation. Analysis of diurnal variations show that tidal volume and minute ventilation are significantly greater during the day than at night in all subjects studied to date but there are no consistent within-subject day-night !differences in breathing frequency. By contrast, the within-subject coefficient of variation (sd/mean) of breathing frequency is significantly greater during the day than at night, while tidal volume and minute ventilation show no consistent day-night differences in this measure of variability. These observations suggest differential physiological control of rate and tidal volume and provide the basis for continued investigation of respiratory correlation of variations in blood pressure response.